Published on Web 03/20/2002
Retention of Configuration on the Oxidative Addition of P-H Bond to Platinum (0) Complexes: The First Straightforward Synthesis of Enantiomerically Pure P-Chiral Alkenylphosphinates via Palladium-Catalyzed Stereospecific Hydrophosphinylation of Alkynes Li-Biao Han,* Chang-Qiu Zhao, Shun-ya Onozawa, Midori Goto, and Masato Tanaka* National Institute of AdVanced Industrial Science and Technology, AIST Tsukuba Central 5, Tsukuba, Ibaraki 305-8565, Japan Received February 5, 2002
P-chiral organophosphorus compounds are of great importance in biological chemistry, organic synthesis, and asymmetric catalysis.1 However, their preparations usually require tedious steps.2 We have reported a series of metal-catalyzed addition reactions of P(V)-H bonds to carbon-carbon unsaturated bonds (hydrophosphinylation), which cleanly and efficiently furnish a wide variety of organophosphorus compounds in high yields and selectivities.3 On the other hand, (RP)-phenylphosphinate 1, a white solid easy to prepare and handle, has been known for more than 30 years.4 With these in mind, we commenced our study on a new synthetic strategy for P-chiral phosphorus compounds via metal-catalyzed stereospecific hydrophosphinylation (eq 1). Since the metal-catalyzed hydrophosphinylation of alkynes is triggered by the oxidative addition of the P-H bond as illustrated in eq 1, stereospecific oxidative addition to the metal (either complete retention or inversion) is one of the preconditions. Although oxidative additions of P-H bonds to metal-complexes have been documented previously, nothing is known about its stereochemistry.3 Herein reported are (1) the first investigation on the stereochemistry of the oxidative addition of the P(V)-H bond to metal complexes, revealing retention of configuration at the phosphorus center in the reaction of 1 with platinum (0) complexes, and (2) the first straightforward synthesis of 2 by a novel palladium-catalyzed stereospecific hydrophosphinylation of alkynes with 1. Compound 2 is a versatile intermediate, which can be readily converted to other P-chiral compounds by simple operations.2
Chart 1
Chart 1). The oxidative addition of the P-H bond to platinum (0), therefore, proceeds stereospecifically. Although complex 3a failed to give crystals suitable for X-ray analysis, the structure of a similar complex 3b, generated via the reaction of 1 with Pt(dcpe)(cod) (dcpe ) 1,2-bis(dicyclohexylphosphino)ethane; cod ) 1,5-cyclooctadiene) has been unambiguously determined (Figure 1).6 The absolute configuration at P(O) in 3b is S, conclusively showing that the oxidatiVe addition of 1 to platinum (0) proceeds with retention of configuration.
Figure 1. Molecular structure of complex 3b.
On the basis of this finding, the metal-catalyzed stereospecific hydrophosphinylation of 1 with alkynes is successfully developed.7 Thus, extensive screening of the reaction procedure has revealed that Me2Pd(PPhMe2)2/Ph2P(O)OH is the right catalyst8 for the addition of 1 to 1-octyne to produce R-adduct 2a in a high yield and regioselectivity (eq 2).9 When an equimolar mixture of 1 and Pt(PEt3)4 was mixed in toluene at room temperature for 1 h, the starting materials completely disappeared to afford white hydrido platinum complex 3a in 87% isolated yield as single stereoisomer.5 On the other hand, a similar treatment of a 50/50 mixture of 1/1′ resulted in the formation of a 50/50 mixture of two isomeric complexes, 3a and 3a′, which are readily distinguishable in NMR spectroscopies5(see * Author for correspondence. E-mail:
[email protected].
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While 2a′ was not found in this particular reaction of 1, a 50/50 mixture of 2a/2a′ was generated by using a 50/50 mixture of 1/1′.10 10.1021/ja025816+ CCC: $22.00 © 2002 American Chemical Society
COMMUNICATIONS Table 1. Stereospecific Hydrophosphinylation of Alkynesa
the corresponding ferrocene, which can be readily applicable in the synthesis of novel optically active ferreocenyl phosphines of great applications in modern asymmetric catalysis.11 As exemplified by run 14, two phosphinyl groups were easily introduced into diynes such as nona-1,8-diyne. Although a relatively prolonged heating was needed, an internal alkyne tolane was also successfully hydrophosphinylated to give the corresponding trans-adduct selectively. In conclusion, both the oxidative additionof the P(V)-H bond to platinum (0) complexes and the palladium-catalyzed hydrophosphinylation of alkynes proceed stereospecifically with retention of configurations at phosphorus, and the latter provides a convenient and general synthetic route to enantiomerically pure P-chiral alkenylphosphinates.12 Further studies on the applications of this finding are now in progress. Acknowledgment. Partial financial support from the Japan Science and Technology Corporation (JST) through the CREST (Core Research for Evolutional Science and Technology) program and a STA fellowship to C.-Q.Z. Supporting Information Available: Spectral and analytical data of products (PDF) and X-ray crystallographic files (CIF). This material is available free of charge via the Internet at http://pubs.acs.org. References
a R ) (-)-menthyl. Conditions: equimolar 1 and an alkyne in toluene (0.4 M), 3 mol % Me2Pd(PPhMe2)2, 5 mol % Ph2P(O)OH, 70 °C, 4.5 h. Regioselectivity to the adduct shown is ca. 95∼98%. b The absolute configuration at phosphorus is RP. (RP/SP g 99). c 1 atm acetylene gas, THF, 67 °C, 15 h. d 2.1 equiv of 1 was used. e trans isomer only. f THF, 67 °C, 16 h.
Since 2a was an oil, we were unable to determine the absolute configuration by X-ray analysis. However, its analogue adduct obtained from the reaction of 1 with 2,4,6-trimethylphenylacetylene afforded a crystalline material, X-ray analysis of which revealed that the configuration was R,10 showing that the palladium-catalyzed hydrophospinylation also proceeds with retention of configuration at phosphorus. The palladium-catalyzed stereospecific hydrophosphinylation has a wide generality. As shown in Table 1, acetylene gas can also be employed as the substrate and the novel synthetically versatile (RP)vinylphosphinate2 was obtained in 93% yield. With the exception of trimethylsilylacetylene3a which gave the β-trans-adduct, other alkynes of both aliphatic and aromatic ones, including those bearing functionalities such as chloro, cyano, carbonylalkoxy, olefinic, and alkoxy groups were all able to react in a efficient way under similar reaction conditions to afford the corresponding (RP)-vinylphosphinates in high yields and selectivities. Note that ethynylferrocene was also successfully hydrophosphinylated to give a good yield of
(1) (a) Quin, L. D. A Guide to Organophosphorus Chemistry; WileyInterscience: New York, 2000. (b) Sasaki, M. In Chirality in Agrochemicals; Kurihara, N., Miyamoto, J., Eds.; Wiley & Sons: Chichester, 1998; pp 85-139. (c) Imamoto, T. In Handbook of Organophosphorus Chemistry; Engel, R., Ed.; Marcel Dekker: New York, 1992; Chapter 1. (d) Kagan H. B.; Sasaki, M. In Chemistry of Organophosphorus Compounds; Hartley, F. R., Ed.; Wiley & Sons: New York, 1990; Vol. 1, Chapter 3. (2) Pietrusiewicz, K. M.; Zablocka, M. Chem. Rew. 1994, 94, 1375. (3) For a review, see: (a) Han, L.-B.; Tanaka, M. Chem. Commun. 1999, 395. For recent examples, see: (b) Han, L.-B.; Mirzaei, F.; Zhao, C.-Q.; Tanaka, M. J. Am. Chem. Soc. 2000, 122, 5407. (c) Zhao, C.-Q.; Han, L.-B.; Goto, M.; Tanaka, M. Angew. Chem., Int. Ed. 2001, 40, 1929. (d) Han, L.-B.; Zhao, C.-Q.; Tanaka, M. J. Org. Chem. 2001, 66, 5929. (4) Pure (RP)-1 (1/1′ g 99/1) was readily prepared in a large scale by adopting a modified method of Mislow. Farnham, W. B.; Murray, R. K.; Mislow, K. J. Am. Chem. Soc. 1970, 92, 5809. See Supporting Information for its characterization. (5) Characteristic signals of 3a and 3a′ in 1H and 31P NMR spectroscopies: 3a, δ -5.23 (Pt-H, JHPt ) 720.4 Hz); δ 100.1 (P(O), JP(O)Pt ) 2803.1 Hz). 3a′, δ -5.53 (Pt-H, JHPt ) 714.9 Hz); δ 104.2 (P(O), JP(O)Pt ) 2800.4 Hz). See Supporting Information for details. (6) Selected bond lengths (Å) and angles (deg) of 3b: O(1)-P(1) ) 1.508(5), O(2)-P(1) ) 1.628(6), P(1)-Pt ) 2.299(2), P(2)-Pt ) 2.304(2), P(3)Pt ) 2.286(2); P(1)-Pt-P(2) ) 97.29(7), P(1)-Pt-P(3) ) 175.31(8), P(2)-Pt-P(3) ) 86.94(7). See Supporting Information for details. (7) Although metal-catalyzed hydrophosphinylations have been studied with (RO)2P(O)H and Ph2P(O)H, which showed different reactivity (ref 3), similar reactions have not been performed with (RO)R′P(O)H yet. (8) Han, L.-B.; Hua, R.; Tanaka, M. Angew. Chem., Int. Ed. 1998, 37, 94. (9) Adduct 2a and its minor β-regioisomer were isolated in pure form and fully characterized. See Supporting Information. (10) Adducts 2a and 2a′ were readily distinguished in 1H NMR spectroscopy. 2a (dCH2): δ 5.71 (d, JHP ) 44.1 Hz), 5.91 (d, JHP ) 21.3 Hz); 2a′ (dCH2) δ 5.58 (d, JHP ) 42.2 Hz), 6.18 (d, JHP ) 20.0 Hz). See Supporting Information. (11) Togni, A.; Hayashi, T. Ferrocenes; VCH: Weinheim, 1995. (12) In the presence of a base, palladium-catalyzed couplings of diastereomerically pure P-H compounds with halides have been reported to give highly pure P-chiral compounds. See (a) Xu, Y.; Wei, H.; Zhang, J.; Huang, G. Tetrahedron Lett. 1989, 30, 949. (b) Oshiki, T.; Imamoto, T. J. Am. Chem. Soc. 1992, 114, 3975. However, an investigation on the scope and limitations of these methods seem to be necessary considering the results of a recent report. (c) Al-Masum, M.; Kumaraswamy, G.; Livinghouse, T. J. Org. Chem. 2000, 65, 4776.
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